1. Field of the Invention
This invention generally relates to an audio signal interpolation method and device, and more particularly to an audio signal interpolation method and device adapted to improve the sound quality by interpolating the skipped spectral components to an audio signal in which some spectral components are skipped.
2. Description of the Related Art
In recent years, the service of digital distribution of music through the Internet is spreading quickly. Usually, in this music distribution service, compression and distribution of an audio signal is commonly performed using the audio coding technique, such as AAC (Advanced Audio Coding) or MP3 (MPEG1 Audio Layer 3).
The above-mentioned audio coding technique of AAC or MP3 is characterized by compressing the audio signal by skipping the spectral components that are not important for the hearing based on the subjectivity of the human being. FIG. 1A shows the frequency spectrum before encoding, and FIG. 1B shows the frequency spectrum after encoding. Suppose that the spectral components which are indicated by the dotted lines in FIG. 1B are skipped.
In this specification, as shown in FIG. 1A and FIG. 1B, the whole audio signal which is expressed by the amplitude levels of respective frequencies will be referred to as frequency spectrum, and the amplitude level of each frequency will be referred to as a spectral component.
Skipping of these spectral components is performed on the basis of a frame which is a collection of audio signal for a plurality of samples, and which spectral components are skipped is determined independently for every frame.
For example, in the encoded spectrum of the frame at the time instant t, the spectral component indicated by the dotted line in FIG. 2A is not skipped, whereas, in the encoded spectrum of the frame at the time instant (t+1), the spectral component indicated by the dotted line in FIG. 2B is skipped. Thus, the phenomenon in which the spectral components move violently may arise.
Since the hearing of the human being is very sensitive to movement of spectral components, the movement of spectral components induces to the human hearing the sense of incongruity. And this causes the sound quality to deteriorate. In order to prevent the deteriorating of the sound quality due to the skipping of spectral components, it is demanded to provide a method of interpolating the skipped spectral components appropriately.
For example, Japanese Patent No. 3576936 discloses a method of interpolating the skipped spectral components. In the method of Japanese Patent No. 3576936, a band where a spectral component does not exist is determined as the band to be interpolated. Then the determined band is interpolated using the spectral components of a corresponding band in the preceding or following frame which is, equivalent to the determined band, or the spectral components of a low-frequency-side band adjacent to the determined band.
FIG. 3A shows the frequency spectrum before interpolation and FIG. 3B shows the way the determined band is interpolated using the spectral components of a low-frequency-side band adjacent to the determined band.
In the conventional method mentioned above, the interpolation is performed by determining a band where a spectral component does not exist as the band to be interpolated. However, there may be two kinds of band where a spectral component does not exist: the skipped band in which spectral components are skipped by the encoding; and the vacancy band in which a spectral component does not exist primarily. Although the skipped band is a band which should be interpolated, the vacancy band is a band which must not be interpolated.
However, in the case of the above-mentioned conventional method, both the skipped band and the vacancy band may be interpolated. Thus, there is a problem that the sound quality will deteriorate because the unnecessary interpolation is performed with respect to the vacancy band where a spectral component does not exist primarily.